Of the many antibiotics described in the prior art, some possess the feature of consisting of several active components acting together synergistically. This is the case, in particular, with the antibiotics of the streptogramin family, which are composed of a macrolactone (group A component) and a depsipeptide (group B component). The antimicrobial activity and the mode of action of these antibiotics have been the subject of studies, in particular by Tanaka (Antibiotics, vol. III p. 487, Springer, Berlin (1975)) and Vasquez (Antibiotics, vol. III p. 521, Springer, Berlin (1975)). The general structure of the molecules of groups A and B is shown in FIGS. 1 and 2.
Among the known antibiotics of the streptogramin family, there may be mentioned, more especially:
pristinamycins, PA1 mikamycins, PA1 virginiamycins, PA1 vernamycins, PA1 ostreogrycins, PA1 synergistins, PA1 plauracins, PA1 etamycins, PA1 streptogramins, PA1 viridogriseins, PA1 griseoviridins, or PA1 neoviridogriseins. PA1 Y represents a D-proline, a 4,5-dehydroproline, a D-alanine or a D-cysteine, PA1 R0 represents a C.dbd.O or CHOH group, PA1 R1 is a hydrogen atom or a methyl group, PA1 R2 is a hydrogen atom or a methyl group, and PA1 R3 is a methyl or isopropyl group. PA1 Z represents an L-proline, an L-aspartic acid, a pipecolic acid, a 4-oxopipecolic acid, a 4-hydroxy-L-pipecolic acid or a 5-hydroxy-4-oxo-L-pipecolic acid, PA1 R4 is a hydrogen atom or an amine of formula NH(CH3) or N(CH3)2, PA1 R5 is a hydrogen atom or a methyl group, and PA1 R6 is a methyl or ethyl group. PA1 R7 represents a hydrogen atom or a hydroxyl group, and PA1 R8 represents a methyl or ethyl group. PA1 R4 is an amine of formula N(CH3)2, R5 is a methyl group, R6 is an ethyl group and Z represents a 4-oxopipecolic acid; or PA1 R4 is an amine of formula NHCH3, R5 is a methyl group, R6 is an ethyl group and Z represents a 4-oxopipecolic acid; or PA1 R4 is an amine of formula N(CH3)2, R5 is a methyl group, R6 is a methyl group and Z represents a 4-oxopipecolic acid; or PA1 R4 is an amine of formula N(CH3)2, R5 is a methyl group, R6 is an ethyl group and Z represents a 5-hydroxy-4-oxo-L-pipecolic acid; or PA1 R4 is a hydrogen atom, R5 is a methyl group, R6 is an ethyl group and Z represents a 4-oxopipecolic acid; or PA1 R4 is a hydrogen atom, R5 is a methyl group, R6 is a methyl group and Z represents a 4-oxopipecolic acid; or PA1 R4 is a hydrogen atom, R5 is a methyl group, R6 is an ethyl group and Z represents a 5-hydroxy-4-oxo-L-pipecolic acid; or PA1 R4 is a hydrogen atom, R5 is a hydrogen atom, R6 is an ethyl group and Z represents a 4-hydroxy-L-pipecolic acid. PA1 in a first, optional step, a mutagenesis is performed on a microorganism that is a non-selective producer of streptogramins, and PA1 in a second step, the selective microorganisms are selected. PA1 physical agents: X-rays, ultraviolet rays; or PA1 chemical agents such as: PA1 any system of mutational insertion into DNA, and especially transposons, integrative plasmids, phages or prophages; or alternatively PA1 protoplast fusion (Cohen, Nature 268 (1977) 171-174).
The active form of each of these antibiotics consists of a synergistic combination of molecules belonging to group A as shown in FIG. 1 and molecules belonging to group B as shown in FIG. 2, or of related molecules.
Related molecules are understood, in the sense used in the present invention, to mean molecules possessing the general skeleton of those shown in FIGS. 1 and 2 but which may differ from the latter by substitutions or other secondary variations, and possessing an activity of the same nature.
The antibiotics of the streptogramin family are produced by a wide variety of microorganisms, and especially by bacteria of the genus Actinomycete and by certain fungi. These microorganisms are characterised in that they synthesise both components A and B simultaneously.
Table 1 lists the main productive microorganisms, together with the corresponding antibiotics.
Studies have been carried out with the object of increasing the levels of production of these microorganisms. There may be mentioned, in particular, the works of Biot (Biotechnology of Industrial Antibiotics, vol. 22 (1984) p. 695) or of Prikrylova et al. (Biotechnology and Bioindustry/2 (1988) 20) concerning improvement in the culture conditions and the effect of mutagenic agents on the levels of production of virginiamycin by S. virginiae. As the authors state, the overproductive mutants obtained always produce components A and B simultaneously.
However, the fact that the streptogramin-producing microorganisms which are available in the prior art synthesise both synergistic components A and B of the antibiotic simultaneously, in the same fermentation medium, constitutes a considerable drawback in some cases.
In effect, to optimise the use of these antibiotics and to be able to use them as pharmaceutical agents, it is preferable to be able to separate and purify the A and B components of the streptogramins. This is also essential in order to be able to carry out chemical studies on streptogramins, especially with the object of preparing semisynthetic derivatives such as, for example, those described in Patents FR 2,549,063, FR 2,549,065, EP 191,662 or EP 248,703.
However, accessibility to these different components is difficult as a result, in particular, of their simultaneous production and of the similarity of their physicochemical properties. Furthermore, these microorganisms generally synthesise several different molecules of each component, leading to a mixture of many compounds, in highly variable proportions, in the fermentation must. In effect, many molecules belonging to groups A and B of streptogramins, possessing very different biological activities and which are produced simultaneously by the microorganisms, are known at the present time. Thus, the following molecules belonging to group A are known in the prior art: pristinamycins PIIA and PIIB, virginiamycins M1 and M2, mikamycin A or ostreogrycins A and G. Likewise, the following molecules belonging to group B are known in the prior art: pristinamycins PIA, PIB and PIC, virginiamycins S1, S2, S3, S4 and S5, mikamycin B, vernamycins B.alpha., B.beta., B.gamma. and B.delta., ostreogrycin B, B1, B2 and B3 or neoviridogriseins I, II, III and IV.
For these reasons, it is difficult to obtain active and pharmaceutically acceptable synergistic mixtures of streptogramins satisfactorily at industrial level. It is also difficult to carry out chemical studies on streptogramins (structure-function relationship, development of water-soluble forms, and the like). The present invention enables the drawbacks of the prior art in this field to be remedied.